Reflective electrophoretic displays including photo-luminescent material and color filter arrays

ABSTRACT

An electro-optic display is provided that may include a layer of light-transmissive conductive material, a substrate, a layer of an electro-optic medium disposed between the layer of conductive material and the substrate, a color filter array, and a light emitting layer. The electro-optic medium may include a photo-luminescent material that functions as either a down-converter or an up-converter that may be excited by the light received from the light emitting layer. The photo-luminescent material may be excited by radiation having a first wavelength transmitted by a filter within the color filter array and emit radiation having a second wavelength transmitted by the filter. The photo-luminescent material may also be excited by radiation at a wavelength within a first and second range of wavelengths transmitted by two filters within the color filter array and emit radiation at a wavelength within one of the first and second ranges.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.16/281,530, filed Feb. 21, 2019, which claims priority to and thebenefit of U.S. Provisional Application Ser. No. 62/640,813, filed onMar. 9, 2018. All patents and publications disclosed herein areincorporated by reference herein in their entireties.

FIELD OF THE INVENTION

This invention relates to reflective displays that include layers ofelectrophoretic media containing photo-luminescent material and colorfilter arrays.

BACKGROUND OF INVENTION

The term “electro-optic”, as applied to a material or a display, is usedherein in its conventional meaning in the imaging art to refer to amaterial having first and second display states differing in at leastone optical property, the material being changed from its first to itssecond display state by application of an electric field to thematerial. Although the optical property is typically color perceptibleto the human eye, it may be another optical property, such as opticaltransmission, reflectance, luminescence, or, in the case of displaysintended for machine reading, pseudo-color in the sense of a change inreflectance of electromagnetic wavelengths outside the visible range.

The term “gray state” is used herein in its conventional meaning in theimaging art to refer to a state intermediate two extreme optical statesof a pixel, and does not necessarily imply a black-white transitionbetween these two extreme states. For example, several of the E Inkpatents and published applications referred to below describeelectrophoretic displays in which the extreme states are white and deepblue, so that an intermediate “gray state” would actually be pale blue.Indeed, as already mentioned, the change in optical state may not be acolor change at all. The terms “black” and “white” may be usedhereinafter to refer to the two extreme optical states of a display, andshould be understood as normally including extreme optical states whichare not strictly black and white, for example the aforementioned whiteand dark blue states. The term “monochrome” may be used hereinafter todenote a drive scheme which only drives pixels to their two extremeoptical states with no intervening gray states.

Some electro-optic materials are solid in the sense that the materialshave solid external surfaces, although the materials may, and often do,have internal liquid- or gas-filled spaces. Such displays using solidelectro-optic materials may hereinafter for convenience be referred toas “solid electro-optic displays”. Thus, the term “solid electro-opticdisplays” includes rotating bichromal member displays, encapsulatedelectrophoretic displays, microcell electrophoretic displays andencapsulated liquid crystal displays.

The terms “bistable” and “bistability” are used herein in theirconventional meaning in the art to refer to displays comprising displayelements having first and second display states differing in at leastone optical property, and such that after any given element has beendriven, by means of an addressing pulse of finite duration, to assumeeither its first or second display state, after the addressing pulse hasterminated, that state will persist for at least several times, forexample at least four times, the minimum duration of the addressingpulse required to change the state of the display element. It is shownin U.S. Pat. No. 7,170,670 that some particle-based electrophoreticdisplays capable of gray scale are stable not only in their extremeblack and white states but also in their intermediate gray states, andthe same is true of some other types of electro-optic displays. Thistype of display is properly called “multi-stable” rather than bistable,although for convenience the term “bistable” may be used herein to coverboth bistable and multi-stable displays.

Several types of electro-optic displays are known. One type ofelectro-optic display is a rotating bichromal member type as described,for example, in U.S. Pat. Nos. 5,808,783; 5,777,782; 5,760,761;6,054,071 6,055,091; 6,097,531; 6,128,124; 6,137,467; and 6,147,791(although this type of display is often referred to as a “rotatingbichromal ball” display, the term “rotating bichromal member” ispreferred as more accurate since in some of the patents mentioned abovethe rotating members are not spherical). Such a display uses a largenumber of small bodies (typically spherical or cylindrical) which havetwo or more sections with differing optical characteristics, and aninternal dipole. These bodies are suspended within liquid-filledvacuoles within a matrix, the vacuoles being filled with liquid so thatthe bodies are free to rotate. The appearance of the display is changedby applying an electric field thereto, thus rotating the bodies tovarious positions and varying which of the sections of the bodies isseen through a viewing surface. This type of electro-optic medium istypically bistable.

Another type of electro-optic display uses an electrochromic medium, forexample an electrochromic medium in the form of a nanochromic filmcomprising an electrode formed at least in part from a semi-conductingmetal oxide and a plurality of dye molecules capable of reversible colorchange attached to the electrode; see, for example O'Regan, B., et al.,Nature 1991, 353, 737; and Wood, D., Information Display, 18(3), 24(March 2002). See also Bach, U., et al., Adv. Mater., 2002, 14(11), 845.Nanochromic films of this type are also described, for example, in U.S.Pat. Nos. 6,301,038; 6,870,657; and 6,950,220. This type of medium isalso typically bistable.

Another type of electro-optic display is an electro-wetting displaydeveloped by Philips and described in Hayes, R. A., et al., “Video-SpeedElectronic Paper Based on Electrowetting”, Nature, 425, 383-385 (2003).It is shown in U.S. Pat. No. 7,420,549 that such electro-wettingdisplays can be made bistable.

One type of electro-optic display, which has been the subject of intenseresearch and development for a number of years, is the particle-basedelectrophoretic display, in which a plurality of charged particles movethrough a fluid under the influence of an electric field.Electrophoretic displays can have attributes of good brightness andcontrast, wide viewing angles, state bistability, and low powerconsumption when compared with liquid crystal displays. Nevertheless,problems with the long-term image quality of these displays haveprevented their widespread usage. For example, particles that make upelectrophoretic displays tend to settle, resulting in inadequateservice-life for these displays.

As noted above, electrophoretic media require the presence of a fluid.In most prior art electrophoretic media, this fluid is a liquid, butelectrophoretic media can be produced using gaseous fluids; see, forexample, Kitamura, T., et al., “Electrical toner movement for electronicpaper-like display”, IDW Japan, 2001, Paper HCS1-1, and Yamaguchi, Y.,et al., “Toner display using insulative particles chargedtriboelectrically”, IDW Japan, 2001, Paper AMD4-4). See also U.S. Pat.Nos. 7,321,459 and 7,236,291. Such gas-based electrophoretic mediaappear to be susceptible to the same types of problems due to particlesettling as liquid-based electrophoretic media, when the media are usedin an orientation which permits such settling, for example in a signwhere the medium is disposed in a vertical plane. Indeed, particlesettling appears to be a more serious problem in gas-basedelectrophoretic media than in liquid-based ones, since the lowerviscosity of gaseous suspending fluids as compared with liquid onesallows more rapid settling of the electrophoretic particles.

Numerous patents and applications assigned to or in the names of theMassachusetts Institute of Technology (MIT), E Ink Corporation, E InkCalifornia, LLC. and related companies describe various technologiesused in encapsulated and microcell electrophoretic and otherelectro-optic media. Encapsulated electrophoretic media comprisenumerous small capsules, each of which itself comprises an internalphase containing electrophoretically-mobile particles in a fluid medium,and a capsule wall surrounding the internal phase. Typically, thecapsules are themselves held within a polymeric binder to form acoherent layer positioned between two electrodes. In a microcellelectrophoretic display, the charged particles and the fluid are notencapsulated within microcapsules but instead are retained within aplurality of cavities formed within a carrier medium, typically apolymeric film. The technologies described in these patents andapplications include:

-   -   (a) Electrophoretic particles, fluids and fluid additives; see        for example U.S. Pat. Nos. 7,002,728 and 7,679,814;    -   (b) Capsules, binders and encapsulation processes; see for        example U.S. Pat. Nos. 6,922,276 and 7,411,719;    -   (c) Microcell structures, wall materials, and methods of forming        microcells; see for example U.S. Pat. Nos. 7,072,095 and        9,279,906;    -   (d) Methods for filling and sealing microcells; see for example        U.S. Pat. Nos. 7,144,942 and 7,715,088;    -   (e) Films and sub-assemblies containing electro-optic materials;        see for example U.S. Pat. Nos. 6,825,829; 6,982,178; 7,112,114;        7,158,282; 7,236,292; 7,443,571; 7,513,813; 7,561,324;        7,636,191; 7,649,666; 7,728,811; 7,729,039; 7,791,782;        7,839,564; 7,843,621; 7,843,624; 8,034,209; 8,068,272;        8,077,381; 8,177,942; 8,390,301; 8,482,835; 8,786,929;        8,830,553; 8,854,721; 9,075,280; and 9,238,340; and U.S. Patent        Applications Publication Nos. 2007/0237962; 2009/0109519;        2009/0168067; 2011/0164301; 2014/0115884; and 2014/0340738;    -   (f) Backplanes, adhesive layers and other auxiliary layers and        methods used in displays; see for example U.S. Pat. Nos.        7,116,318 and 7,535,624;    -   (g) Color formation and color adjustment; see for example U.S.        Pat. Nos. 6,017,584; 6,545,797; 6,664,944; 6,788,452; 6,864,875;        6,914,714; 6,972,893; 7,038,656; 7,038,670; 7,046,228;        7,052,571; 7,075,502; 7,167,155; 7,385,751; 7,492,505;        7,667,684; 7,684,108; 7,791,789; 7,800,813; 7,821,702;        7,839,564; 7,910,175; 7,952,790; 7,956,841; 7,982,941;        8,040,594; 8,054,526; 8,098,418; 8,159,636; 8,213,076;        8,363,299; 8,422,116; 8,441,714; 8,441,716; 8,466,852;        8,503,063; 8,576,470; 8,576,475; 8,593,721; 8,605,354;        8,649,084; 8,670,174; 8,704,756; 8,717,664; 8,786,935;        8,797,634; 8,810,899; 8,830,559; 8,873,129; 8,902,153;        8,902,491; 8,917,439; 8,964,282; 9,013,783; 9,116,412;        9,146,439; 9,164,207; 9,170,467; 9,170,468; 9,182,646;        9,195,111; 9,199,441; 9,268,191; 9,285,649; 9,293,511;        9,341,916; 9,360,733; 9,361,836; 9,383,623; and 9,423,666; and        U.S. Patent Applications Publication Nos. 2008/0043318;        2008/0048970; 2009/0225398; 2010/0156780; 2011/0043543;        2012/0326957; 2013/0242378; 2013/0278995; 2014/0055840;        2014/0078576; 2014/0340430; 2014/0340736; 2014/0362213;        2015/0103394; 2015/0118390; 2015/0124345; 2015/0198858;        2015/0234250; 2015/0268531; 2015/0301246; 2016/0011484;        2016/0026062; 2016/0048054; 2016/0116816; 2016/0116818; and        2016/0140909;    -   (h) Methods for driving displays; see for example U.S. Pat. Nos.        7,012,600 and 7,453,445;    -   (i) Applications of displays; see for example U.S. Pat. Nos.        7,312,784 and 8,009,348; and    -   (j) Non-electrophoretic displays, as described in U.S. Pat. No.        6,241,921 and U.S. Patent Application Publication No.        2015/0277160; and applications of encapsulation and microcell        technology other than displays; see for example U.S. Patent        Application Publications Nos. 2015/0005720 and 2016/0012710.

Many of the aforementioned patents and applications recognize that thewalls surrounding the discrete microcapsules in an encapsulatedelectrophoretic medium could be replaced by a continuous phase, thusproducing a so-called polymer-dispersed electrophoretic display, inwhich the electrophoretic medium comprises a plurality of discretedroplets of an electrophoretic fluid and a continuous phase of apolymeric material, and that the discrete droplets of electrophoreticfluid within such a polymer-dispersed electrophoretic display may beregarded as capsules or microcapsules even though no discrete capsulemembrane is associated with each individual droplet; see for example,the aforementioned U.S. Pat. No. 6,866,760. Accordingly, for purposes ofthe present application, such polymer-dispersed electrophoretic mediaare regarded as sub-species of encapsulated electrophoretic media.

Although electrophoretic media are often opaque (since, for example, inmany electrophoretic media, the particles substantially blocktransmission of visible light through the display) and operate in areflective mode, many electrophoretic displays can be made to operate ina so-called “shutter mode” in which one display state is substantiallyopaque and one is light-transmissive. See, for example, U.S. Pat. Nos.5,872,552; 6,130,774; 6,144,361; 6,172,798; 6,271,823; 6,225,971; and6,184,856. Dielectrophoretic displays, which are similar toelectrophoretic displays but rely upon variations in electric fieldstrength, can operate in a similar mode; see U.S. Pat. No. 4,418,346.Other types of electro-optic displays may also be capable of operatingin shutter mode. Electro-optic media operating in shutter mode may beuseful in multi-layer structures for full color displays; in suchstructures, at least one layer adjacent the viewing surface of thedisplay operates in shutter mode to expose or conceal a second layermore distant from the viewing surface.

An encapsulated electrophoretic display typically does not suffer fromthe clustering and settling failure mode of traditional electrophoreticdevices and provides further advantages, such as the ability to print orcoat the display on a wide variety of flexible and rigid substrates.(Use of the word “printing” is intended to include all forms of printingand coating, including, but without limitation: pre-metered coatingssuch as patch die coating, slot or extrusion coating, slide or cascadecoating, curtain coating; roll coating such as knife over roll coating,forward and reverse roll coating; gravure coating; dip coating; spraycoating; meniscus coating; spin coating; brush coating; air knifecoating; silk screen printing processes; electrostatic printingprocesses; thermal printing processes; ink jet printing processes;electrophoretic deposition (See U.S. Pat. No. 7,339,715); and othersimilar techniques.) Thus, the resulting display can be flexible.Further, because the display medium can be printed (using a variety ofmethods), the display itself can be made inexpensively.

Other types of electro-optic media may also be used in the displays ofthe present invention.

An electrophoretic display normally comprises a layer of electrophoreticmaterial and at least two other layers disposed on opposed sides of theelectrophoretic material, one of these two layers being an electrodelayer. In most such displays both the layers are electrode layers, andone or both of the electrode layers are patterned to define the pixelsof the display. For example, one electrode layer may be patterned intoelongate row electrodes and the other into elongate column electrodesrunning at right angles to the row electrodes, the pixels being definedby the intersections of the row and column electrodes. Alternatively,and more commonly, one electrode layer has the form of a singlecontinuous electrode and the other electrode layer is patterned into amatrix of pixel electrodes, each of which defines one pixel of thedisplay. In another type of electrophoretic display, which is intendedfor use with a stylus, print head or similar movable electrode separatefrom the display, only one of the layers adjacent the electrophoreticlayer comprises an electrode, the layer on the opposed side of theelectrophoretic layer typically being a protective layer intended toprevent the movable electrode damaging the electrophoretic layer.

The manufacture of a three-layer electrophoretic display normallyinvolves at least one lamination operation. For example, in several ofthe aforementioned MIT and E Ink patents and applications, there isdescribed a process for manufacturing an encapsulated electrophoreticdisplay in which an encapsulated electrophoretic medium comprisingcapsules in a binder is coated on to a flexible substrate comprisingindium-tin-oxide (ITO) or a similar conductive coating (which acts asone electrode of the final display) on a plastic film, thecapsules/binder coating being dried to form a coherent layer of theelectrophoretic medium firmly adhered to the substrate. Separately, abackplane, containing an array of pixel electrodes and an appropriatearrangement of conductors to connect the pixel electrodes to drivecircuitry, is prepared. To form the final display, the substrate havingthe capsule/binder layer thereon is laminated to the backplane using alamination adhesive. (A very similar process can be used to prepare anelectrophoretic display usable with a stylus or similar movableelectrode by replacing the backplane with a simple protective layer,such as a plastic film, over which the stylus or other movable electrodecan slide.) In one preferred form of such a process, the backplane isitself flexible and is prepared by printing the pixel electrodes andconductors on a plastic film or other flexible substrate. The obviouslamination technique for mass production of displays by this process isroll lamination using a lamination adhesive.

The aforementioned U.S. Pat. No. 6,982,178 describes a method ofassembling a solid electro-optic display (including an encapsulatedelectrophoretic display) which is well adapted for mass production.Essentially, this patent describes a so-called “front plane laminate”(“FPL”) which comprises, in order, a light-transmissiveelectrically-conductive layer; a layer of a solid electro-optic mediumin electrical contact with the electrically-conductive layer; anadhesive layer; and a release sheet. Typically, the light-transmissiveelectrically-conductive layer will be carried on a light-transmissivesubstrate, which is preferably flexible, in the sense that the substratecan be manually wrapped around a drum (say) 10 inches (254 mm) indiameter without permanent deformation. The term “light-transmissive” isused in this patent and herein to mean that the layer thus designatedtransmits sufficient light to enable an observer, looking through thatlayer, to observe the change in display states of the electro-opticmedium, which will normally be viewed through theelectrically-conductive layer and adjacent substrate (if present); incases where the electro-optic medium displays a change in reflectivityat non-visible wavelengths, the term “light-transmissive” should ofcourse be interpreted to refer to transmission of the relevantnon-visible wavelengths. The substrate will typically be a polymericfilm, and will normally have a thickness in the range of about 1 toabout 25 mil (25 to 634 μm), preferably about 2 to about 10 mil (51 to254 μm). The electrically-conductive layer is conveniently a thin metalor metal oxide layer of, for example, aluminum or ITO, or may be aconductive polymer. Poly(ethylene terephthalate) (PET) films coated withaluminum or ITO are available commercially, for example as “aluminizedMylar” (“Mylar” is a Registered Trade Mark) from E.I. du Pont de Nemours& Company, Wilmington Del., and such commercial materials may be usedwith good results in the front plane laminate.

Assembly of an electro-optic display using such a front plane laminatemay be effected by removing the release sheet from the front planelaminate and contacting the adhesive layer with the backplane underconditions effective to cause the adhesive layer to adhere to thebackplane, thereby securing the adhesive layer, layer of electro-opticmedium and electrically-conductive layer to the backplane. This processis well-adapted to mass production since the front plane laminate may bemass produced, typically using roll-to-roll coating techniques, and thencut into pieces of any size needed for use with specific backplanes.

U.S. Pat. No. 7,561,324 describes a so-called “double release sheet”which is essentially a simplified version of the front plane laminate ofthe aforementioned U.S. Pat. No. 6,982,178. One form of the doublerelease sheet comprises a layer of a solid electro-optic mediumsandwiched between two adhesive layers, one or both of the adhesivelayers being covered by a release sheet. Another form of the doublerelease sheet comprises a layer of a solid electro-optic mediumsandwiched between two release sheets. Both forms of the double releasefilm are intended for use in a process generally similar to the processfor assembling an electro-optic display from a front plane laminatealready described, but involving two separate laminations; typically, ina first lamination the double release sheet is laminated to a frontelectrode to form a front sub-assembly, and then in a second laminationthe front sub-assembly is laminated to a backplane to form the finaldisplay, although the order of these two laminations could be reversedif desired.

U.S. Pat. No. 7,839,564 describes a so-called “inverted front planelaminate”, which is a variant of the front plane laminate described inthe aforementioned U.S. Pat. No. 6,982,178. This inverted front planelaminate comprises, in order, at least one of a light-transmissiveprotective layer and a light-transmissive electrically-conductive layer;an adhesive layer; a layer of a solid electro-optic medium; and arelease sheet. This inverted front plane laminate is used to form anelectro-optic display having a layer of lamination adhesive between theelectro-optic layer and the front electrode or front substrate; asecond, typically thin layer of adhesive may or may not be presentbetween the electro-optic layer and a backplane. Such electro-opticdisplays can combine good resolution with good low temperatureperformance.

Many types of electro-optic media are essentially monochrome, in thesense that any given medium has two extreme optical states and a rangeof gray levels lying between the two extreme optical states. However,there is today an increasing demand for full color displays, even forsmall, portable displays; for example, most displays on cellulartelephones are today full color. To provide a full color display usingmonochrome media, it is either necessary to place a color filter arraywhere the display can be viewed through the color filter array, or toplace areas of different electro-optic media capable of displayingdifferent colors adjacent one another.

The position of the color filter array (CFA) relative to theelectro-optic medium in the optical stack can vary widely, but must takeinto account the type of electro-optic medium used and, in some cases,the properties of other layers of the optical stack. Color displaysusing CFA's can be broadly divided into two classes, namely front CFAdisplays (in which the CFA lies between the electro-optic medium and theviewing surface through which an observer views the display) and rearCFA displays (in which the CFA lies on the opposed side of theelectro-optic medium from the viewing surface). If the electro-opticmedium used is transmissive (i.e., light, typically from a backlight,passes through the electro-optic medium, which acts as a light valvecontrolling the amount of light which passes through each pixel, andthen emerges from the viewing surface), the CFA can occupy any positionin the optical stack, since regardless of the position of the CFA, lightwill pass through both the CFA and the electro-optic medium. Thus, bothfront and rear CFAs can be used with transmissive electro-optic media,although the former are probably more common.

Front CFA displays suffer from the disadvantage that color filter arraysinherently absorb light, and the overall effect of the absorption oflight by a front CFA is a darkening of the white state of the display. Afront CFA also can exhibit unwanted reflection, for example byscattering, and the overall effect of the reflection of light by a frontCFA is a slight desaturation of displayed colors. For example consider afront CFA display in which the color filter comprises regions of equalarea that pass red, green, blue and white light, respectively (a RGBWdisplay). The result is an absorption by the CFA of, in principle, 50%of light that could have been reflected from the display in the whitestate. Moreover if, for example, a particular primary color is intendedto be displayed at maximum saturation, it is necessary for the displaybe switched to absorb light in all regions except those behind theparticular CFA element corresponding to the desired color, so that only25% of the total area of the display is available for display of aparticular primary color. This results in a gamut of rather dark colors.In order to allow more light to pass, spectrally broad color filters maybe selected; however, this will come at the cost of colorfulness, i.e.color saturation.

Accordingly, there is a need for improved electro-optic displays thatutilize CFAs.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention, an electro-opticdisplay is provided that may comprise a layer of light-transmissiveconductive material, a substrate, a layer of an electro-optic mediumdisposed between the layer of light-transmissive conductive material andthe substrate, the electro-optic medium comprising a photo-luminescentmaterial, and a color filter array. The photo-luminescent material maybe excited by radiation at a first wavelength and emit radiation at asecond wavelength, the second wavelength being either longer or shorterthan the first wavelength.

According to another aspect of the present invention, an electro-opticdisplay is provided that may comprise a layer of light-transmissiveconductive material, a substrate, a layer of an electro-optic mediumdisposed between the layer of light-transmissive conductive material andthe substrate, and a color filter array comprising a first color filterconfigured to transmit radiation within a first range of wavelengths anda second color filter configured to transmit radiation with a secondrange of wavelengths. The photo-luminescent material may be excited byradiation at a wavelength within both of the first and second ranges andemit radiation at a wavelength within one of the first and secondranges.

These and other aspects of the present invention will be apparent inview of the following description.

BRIEF DESCRIPTION OF THE FIGURES

The drawing Figures depict one or more implementations in accord withthe present concepts, by way of example only, not by way of limitations.In the figures, like reference numerals refer to the same or similarelements.

FIG. 1 is a cross-section side view of a schematic of a reflectivedisplay according to a first embodiment of the present invention.

FIG. 2 is a graph of the transmittance of radiation at variouswavelengths of a color filter array having red, blue, and, green filtersthat may be incorporated in one embodiment of the present invention.

FIGS. 3A to 3C are graphs illustrating the color filter arraytransmittance, emission and excitation spectra of photo-luminescentmaterial, and returned spectra of a prophetic example of a reflectivedisplay according to another embodiment of the invention.

FIG. 4 is a graph of the transmittance of radiation at wavelengthsextended into the near-infrared of a color filter array having red,blue, and, green filters that may be incorporated in yet anotherembodiment of the present invention.

DETAILED DESCRIPTION

In the following detailed description, numerous specific details are setforth by way of examples in order to provide a thorough understanding ofthe relevant teachings. However, it should be apparent to those skilledin the art that the present teachings may be practiced without suchdetails.

Generally, the various embodiments of the present invention may providean electro-optic display that includes a color filter array and anelectro-optic medium comprising a photo-luminescent material thatfunctions as a down-converter or an up-converter. As used throughout thespecification and the claims, a “down-converter” means aphoto-luminescent substance that absorbs photons and then re-emits themat a typically lower energy, i.e. frequency. As used throughout thespecification and the claims, an “up-converter” means aphoto-luminescent substance that absorbs two photons and then re-emits asingle photon at a higher energy, i.e. frequency.

One or more down-converters or up-converters may be incorporated in theelectro-optic medium of the color display for the purpose of shiftingincident light from wavelengths of less sensitivity of the human visualsystem to those of greater sensitivity, to shift from wavelengths thatare in overlapping ranges of wavelengths of radiation transmitted by twofilters within a CFA into a range of wavelengths of radiationtransmitted by one of the two filters, to sharpen the spectral responseof a filter by shifting from a lower wavelength part of a range wherethe CFA transmits radiation to a higher wavelength part, and/or to shiftthe spectral peak of a within a range of wavelengths of radiationtransmitted by a filter to a different peak that leads to a more optimalcombination of color primaries.

Referring now to FIG. 1 , a reflective display 10 is illustratedaccording to a first embodiment of the present invention. The reflectivedisplay 10 may comprise a color filter array 11 having a plurality ofdifferent color filters, such as a red filter 12 a, green filter 12 b,and blue filter 12 c, for example. Laminated below the color filterarray 11 is a light transmissive conductive layer 13 and a layer ofelectro-optic media 14, preferably in the form of a dispersion 16comprising electrophoretic particles, such as white particles 18 andblack particles 20. A substrate 15 below the layer of electro-opticmedia 14 may include a plurality of electrodes used to drive theparticles 18, 20 between a light state in which the white particles 18are driven to the viewing side of the display and a dark state in whichthe black particles 20 are driven to the viewing side of the display.The dispersion fluid, in addition to containing electrophoreticparticles, may further comprise one or more types of photo-luminescentmaterials (P_(R), P_(G), P_(B)). In a dark state, the reflection ofambient light (A) transmitted through the color filter array 11 isprevented by the black particles 20. In a light state, ambient light (A)transmitted through the color filter array 11 is reflected off of whiteparticles 18 and absorbed by any photo-luminescent material (P_(R),P_(G), P_(B)) and re-emitted. The sum of the reflected and emitted light(B) passes back through the color filter array 11 back to the viewer.

In a most preferred embodiment of an electrophoretic display comprisinga color filter array according to the present invention, charged whiteparticles of electrophoretic dispersion may comprise thephoto-luminescent material. In a less preferred embodiment, thephoto-luminescent material may be incorporated into the dispersion fluidand/or in non-charged white particles of a dispersion. In a leastpreferred embodiment, the photo-luminescent material may be incorporatedinto the dispersion fluid that further comprises charged whiteparticles. The display made according to the present invention mayinclude one type of dispersion fluid. Alternatively, a display mayhaving a plurality of formulations, wherein each formulation may havedifferent combinations of photo-luminescent material on a per-pixelbasis, i.e. the dispersion formulation within the viewing area of onecolor filter may differ from the dispersion formulation within the areaof a different color filter.

The photo-luminescent material may comprise one or more phosphorescentmaterials, fluorescent materials, quantum dots, up-converternanoparticles, or combinations thereof, such as those described in“Phosphors, Up Conversion Nano Particles, Quantum Dots and TheirApplications” Vol. 1 by Ru-Shi Liu (2017), the content of which isincorporated by reference herein in its entirety. The photo-luminescentmaterials preferably have narrow excitation spectra, preferably between50-100 nm full width at half maximum (FWHM), and narrow emission spectra(<100 nm FWHM), with high quantum yields of 50% to more than 90%, and alifetime of less than 100 ms. As used herein throughout thespecification and the claims, the “excitation spectrum” is theabsorptance as a function of wavelength at which photons are absorbed,the “emission spectrum” is the normalized energy emitted per wavelength,the “lifetime” is the average time between absorption and emission, andthe “quantum yield” is the efficiency of the energy transfer fromabsorption to re-emission.

Examples of photo-luminescent materials that may be incorporated intothe various embodiments of the present invention include, but are notlimited to, N-hydroxysuccinimidyl (NHS) esters, such as DyLight® 405,DyLight® 488, DyLight™ 549, DyLight® 649, DyLight® 680, and DyLight® 800manufactured and sold by Thermo Fisher Scientific Inc. of Waltham,Mass., aminomethylcoumarin (AMCA), ATTO 425, ATTO 488, ATTO 594, ATTO532, ATTO 550, ATTO 647N, and ATTO 655 manufactured and sold by Atto-TecGmbH of Siegen, Germany, cyanine dyes, such as Cy2, Cy3, Cy3.5, Cy5,Cy5.5, fluorescein, tetraymethyl rhodamine (TRITC), R-phycoerythrin(RPE), sulforhodamine 101 acid chloride (Texas Red), andallophycocyanin. Upconverting nanoparticles include, but are not limitedto, rare earth doped nanocrystals, consisting of a transparent matrix,for example fluorides, such as NaYF₄, NaGdF₄, LiYF₄, YF₃, CaF₂, oroxides, such as Gd₂O₃, doped with a combination of a trivalentrare-earth sensitizer (e.g. Yb, Nd, Er, or Sm) to absorb NIR radiationand a second lanthanide activator (e.g. Er, Ho, Pr, Tm) ion serving asemitter, manufactured and sold as Sunstone® Upconverting Nanocrystals bySigma-Aldrich, Inc. of St. Louis, Mo. An alternative class ofup-converting materials are semiconducting core-shell nanoparticles withquantum dot-quantum well heterostructures, including, but not limitedto, PbSe core/CdSe shell nanodots [Ayelet Teitelboim and Dan Oron,Broadband Near-Infrared to Visible Upconversion in Quantum Dot-QuantumWell Heterostructures, ACS Nano 2016 10 (1), 446-452, DOI:10.1021/acsnano.5b05329].

According to a first embodiment of the present invention, anelectro-optic display may comprise an electrophoretic fluid containingat least one photo-luminescent material, such as a downconverter, thatabsorbs radiation at or below about 425 nm (i.e. violet-near UV) andemit light that may be sensed by the human visual system. Oftenelectrophoretic displays include protective UV coatings to protect theunderlying layers from damage. If the various embodiments of the presentinvention include an optional UV protective coating, thephoto-luminescent material may be capable of emitting light afterabsorbing radiation of a wavelength that is less than or equal to about425 nm (i.e. violet-near UV) and greater than or equal to the wavelengthof radiation that is able to successfully pass through the UV protectivecoating.

In one example of the first embodiment, an electrophoretic display mayinclude a color filter array that includes a red, blue, and green filterand an optional UV filter. The UV filters may allow the transmission ofsome radiation at or below about 425 nm. If the color filter arrayincludes filters that allow significant transmission in this range, suchas a color filter array that exhibits a transmission graphicallyrepresented in FIG. 2 , a downconverter which absorbs violet and/ornear-UV radiation and emits radiation within the visible spectrum forhumans may be incorporated in the electrophoretic fluid of the display.

If the selected fluorescent material converts violet light to red light,this would have the effect, for example, of both eliminating the violetcontamination of the red pixels (resulting from inefficient absorptionof violet light by the red color filter) and increasing the apparentbrightness of the display (since the human visual system is moresensitive to red than to violet light). In another example, thedownconverter after absorbing radiation at or below about 425 nm (i.e.violet/near UV) may emit radiation at or about 450 nm (i.e. blue light).By this means, the perceived brightness of the blue light would beincreased.

According to a second embodiment of the present invention, anelectro-optic display may comprise a color filter array and anelectrophoretic fluid containing a photo-luminescent material, such as adownconverter, that absorbs radiation at a wavelength that may betransmitted by two filters within the color filter array and emitradiation at a wavelength that is substantially transmitted by only oneof the two filters.

Referring again to FIG. 2 , a color filter array included in anelectro-optic display may have filters that each transmit a range ofradiation wavelengths and a portion of each range of respective filtersmay overlap, such as the transmitted spectra for the blue and greenfilters around 490 nm, for example, and the green and red filters around590 nm, for example. In one example of the second embodiment describedabove, the electrophoretic fluid may include a downconverter that isexcited by radiation at a first wavelength within a range of wavelengthscommon to the two filters and emit radiation at a second wavelengthoutside of the range, for example, a wavelength at which one of thefilters transmits little to no light. In one example a down convertermay be excited at or around 490 nm, such as 470 to 520 nm, (i.e. betweengreen and blue) and have an emission peak around 550 nm, such as 520 to560 nm) (i.e. green). Alternatively in another example of the secondembodiment described above, the electrophoretic fluid may include adownconverter that absorbs radiation around 590 nm, such as 560 to 635nm, (i.e. between green and red) that has an emission peak around 650nm, such as 635 to 700 nm (i.e. red).

According to a third embodiment of the present invention, anelectro-optic display may comprise a color filter array and anelectrophoretic fluid containing a photo-luminescent material, such as adownconverter, that absorbs radiation having a first wavelength that istransmitted by a filter within the color filter array and emit radiationhaving a second wavelength that is transmitted by the filter, the secondwavelength being longer than the first wavelength. By this means thetotal light energy may be conserved (in the perfect quantum yield case),but the reflected spectrum associated with each color filter may besharpened, as illustrated in FIGS. 3A to 3C, which is a propheticexample of a reflective display according to the third embodiment of theinvention.

According to a fourth embodiment of the present invention, anelectro-optic display may comprise a color filter array and anelectrophoretic fluid containing a photo-luminescent material, such asan up-converter, that absorbs radiation having a first wavelength thatis transmitted by a filter within the color filter array and emitradiation at a second wavelength that is transmitted by the filter, thesecond wavelength being shorter than the first wavelength. For example,a photo-luminescent material, such as an up-converter nanoparticle, maybe excited by infrared radiation (e.g. radiation having a wavelengthgreater than about 700 nm) and emit radiation at a wavelength belowabout 700 nm, preferably visible light. This may take advantage of acolor filter array comprising filters having a high transmittance in thenear-infrared range of the spectrum, as illustrated in FIG. 4 . However,it should be noted that current state of the art up-converting materialsmay require high irradiance to function efficiently.

According to a fifth embodiment of the present invention each of theembodiments described above may be combined with a light emitting layer.The light emitting layer incorporated into the reflective display mayemit UV, visible, or IR light. Reflective displays according to thepresent invention may be used indoors or outdoors. For indoorapplications, for example, a light emitting layer may be incorporatedinto the reflective display to provide a constant,environment-independent supply of radiation having the necessarywavelength and intensity to excite the photo-luminescent material. Forambient illumination, the amount of UV (and to a lesser degree of NIR)is highly dependent on the lighting environment. For example, windows inbuildings and vehicles will cut UV, and also NIR if they are of theenergy-efficient type. Incandescent and fluorescent lighting might nothave enough UV to excite the photo-luminescent material in a reflectivedisplay according to some embodiments of the invention. Similarly, thelow end of the spectrum of LED lamps is generally at about 410 nm, whichmay not be an adequate wavelength depending on the selectedphoto-luminescent material. Thus, incorporating a light emitting layerinto a reflective display according to the present invention may allowfor a more constant performance that is independent of the quality ofthe surrounding light.

Referring again to FIG. 1 , the light emitting layer may be incorporatedinto the display, such that the layer of electro-optic medium receiveslight emitted by the light emitting layer and excites thephoto-luminescent material present in the electro-optic medium. Forexample, the light emitting layer may be located above the color filterarray 11, such that the color filter array 11 is between the lightemitting layer and the layer of electro-optic medium 14. Alternatively,the light emitting layer may be placed between the color filter array 11and layer of electro-optic medium 14. As noted above, the light emittedby the light emitting layer may be tuned to the wavelength of light thatwill cause excitation of the photo-luminescent material in theelectro-optic medium. In one example, if the light emitting layer isconfigured to emit UV or NIR radiation to excite the photo-luminescentmaterial, the light will not be perceived by a viewer as “switched on”but its effect will be visible. Alternatively, the light emitting layermay emit visible light having a wavelength transmitted by one or more ofthe filters of the color filter array.

While preferred embodiments of the invention have been shown anddescribed herein, it will be understood that such embodiments areprovided by way of example only. Numerous variations, changes, andsubstitutions will occur to those skilled in the art without departingfrom the spirit of the invention. Accordingly, it is intended that theappended claims cover all such variations as fall within the spirit andscope of the invention.

We claim:
 1. An electro-optic display comprising a layer oflight-transmissive conductive material, a substrate, a layer of anelectro-optic medium disposed between the layer of light-transmissiveconductive material and the substrate, the electro-optic mediumcomprising a photo-luminescent material, and a color filter array,wherein the photo-luminescent material is excited by radiation at afirst wavelength and emits radiation at a second wavelength, the secondwavelength being shorter than the first wavelength.
 2. The electro-opticdisplay of claim 1, wherein the photo-luminescent material absorbs lighthaving a wavelength greater than 700 nm and emits visible light having awavelength less than or equal to 700 nm.
 3. The electro-optic display ofclaim 1, wherein the photo-luminescent material comprises ananoparticle.
 4. The electro-optic display of claim 1, wherein the colorfilter array is between the layer of light-transmissive conductivematerial and the layer of electro-optic material.
 5. The electro-opticdisplay of claim 1, wherein the color filter array is between thesubstrate and the layer of electro-optic material.
 6. The electro-opticdisplay of claim 1, wherein the substrate comprises a plurality ofconductive electrodes.
 7. The electro-optic display of claim 6, whereinthe plurality of conductive electrodes form the color filter array. 8.The electro-optic display of claim 1, wherein the electro-optic mediumfurther comprises a dispersion of electrophoretic particles in anon-polar solvent.
 9. The electro-optic display of claim 8, wherein theelectrophoretic particles comprises a plurality of black particles and aplurality of white particles.
 10. The electro-optic display of claim 9,wherein the white particles comprise the photo-luminescent material. 11.The electro-optic display of claim 9, wherein the pluralities of blackand white particles are charged and the black particles have a chargepolarity that is opposite to a charge polarity of the white particles.12. The electro-optic display of claim 9, wherein the charged whiteparticles comprise the photo-luminescent material.
 13. The electro-opticdisplay of claim 1 further comprising a light emitting layer, whereinthe display is configured such that the photo-luminescent material isexcited by light emitted by the light emitting layer.